The present invention relates to a photoelectric incremental measuring system utilizing a collimated light beam for measuring the relative position of two objects by means of a scale and a scanning plate, both of which comprise a line grid having a predetermined grid spacing, wherein the scale and the scanning plate are mounted to move with respect to one another while maintaining a substantially constant relative spacing.
In one type of incremental length and angle measuring device of the prior art, position dependent electrical signals are generated by means of a scale mounted to move with respect to a scanning plate at a certain spacing. Typically, both the scale and the scanning plate are provided with a line grid having a predetermined grid spacing. Suitable illumination is provided such that movement of the scanning unit with respect to the scale produces fluctuations in the intensity of light directed at the scale, which fluctuations are detected by means of photoelectric transducers such as phototransistors. Electrical signals generated by the photoelectric transducers provide a measure of the relative displacement between the scale and the scanning plate.
In incremental measuring systems of the prior art the scale and the scanning plate are generally provided with grid spacings which are virtually identical. Optimal optical signals are generated only at a certain predetermined distance from the scanning plate, which distance is a function of refraction or reproduction between the scale and the scanning plate. For optimal functioning of the apparatus this distance must be maintained constant within a certain tolerance. When a scale and a scanning plate with the grid spacing P.sub.M are illuminated with collimated light of the wavelength .lambda. the optimum spacing is nP.sub.M.sup.2 /.lambda. (n=0, 1, 2 . . . ) and for the tolerance .+-.(0.1 . . . 0.2) P.sub.M.sup.2 /.lambda. (Machine Shop Magazine, April 1962, page 208).
In West German unexamined patent specification OS No. 1,448,504 there is described a photoelectric length measuring device in which the distance between the grid scale and the grid scanning plate amounts to Ka.sup.2 /.lambda., K being a positive whole number, preferably one, a being the grid spacing, and .lambda. the spectral sensitivity center of gravity of the entire photoelectric arrangement. To achieve a sine or a sin.sup.2 curve for the light distribution behind the grid scanning plate the ratio of the width of an absorbing line to the width of a transparent line of the grid scale is chosen to be appreciably smaller than 1:1, for example 3:7, the grid spacing of the scale being equal to the grid spacing of the scanning plate.
Scales and scanning plates with large grid spacings can be made more easily and more cheaply than those with small grid spacings. In order to achieve higher resolution in the measurement of relative movement between the scale and the scanning plate the electric signals of the photoelectric transducers are often subdivided electronically by a suitable circuit, which circuit is in many cases relatively expensive to manufacture.